The present invention relates to autoignition compositions used in inflator bags of automobile passenger-restraint devices.
In the event of a fire during shipment of a passenger-restraint device, the potential rupture of the pressure vessel is a serious safety concern that has been addressed through utilization of an autoignition compound. When the gas inflator is exposed to fire, the autoignition compound is used to ignite the gas generant of the inflator, thereby preventing rupture and scattered fragmentation of the metallic pressure vessel. Several problems have become apparent when designing an autoignition compound. Although the prior art individually addresses the problems, no autoignition composition has yet provided a combined solution to the various design considerations.
Steel canisters are commonly used as the inflator pressure vessel in a passenger-restraint system because of the relatively high strength of steel at elevated temperatures. Given the emphasis on vehicle weight reduction, it is desirable that metals such as aluminum, and smaller or lighter steel vessels be utilized in the pressure vessel.
Engineering considerations require that vehicle operator restraint systems pass a "bonfire" test, wherein the inflator system is evaluated during exposure to fire. In the past, this has only been a concern for inflator canisters made of aluminum as the current steel pressure vessels routinely pass this test. Aluminum loses strength rapidly with increasing temperature, and may not be able to withstand the combination of increased ambient temperatures and excessive internal temperature and pressure generated upon combustion of the gas generant. An autoignition temperature of 175.degree. C. or less is considered autoignition temperature of 175.degree. C. or less is considered sufficient for the safe use of aluminum canisters.
Although steel pressure vessels do not lose strength as rapidly as aluminum vessels at temperatures above ambient, it is still necessary to ignite and burn the gas generant at a similar temperature due to the high internal pressures created by ignition of the gas generant at high temperatures. At temperatures above 110.degree. C., the possible melting of the fuel component(s) of a mixture as well as the rapid ignition of common gas generants will occur due to high temperatures. The inflator must be designed to maintain its structural integrity despite the high pressures produced by a rapidly burning gas generant. If the gas generant of the inflator can be made to autoignite at relatively low temperatures, for example, 150.degree. C. to 175.degree. C., then the pressure vessel can be made of a lightweight metal.
Another concern is that many nonazide gas generant compositions do not meet the gaseous effluent requirements met by current azide based inflators. The autoignition material, a fraction of the total gas generant, has been found to create excessive levels of undesirable gaseous effluents, particularly carbon monoxide and nitrogen oxides. If an autoignition composition can be developed that produces little or no noxious gases, then the nonazide autoignition compositions will conform more closely to the effluent levels currently achieved by azide fuels.
A further concern involves the industry drive to reduce the size of the inflator by eliminating or reducing the volume of its components. Most inflator systems are deployed by the combustion of a gas generant composition comprising a booster, an autoigniter, and a main gas generant charge. In the event of a collision, electrical initiation of a squib ignites the booster that in turn supplies sufficient energy to ignite the main gas generant charge thereby deploying the gas inflator.
Alternatively, in the absence of an accident, but in the event of a fire during shipment, a separate autoignition composition is placed in close proximity to the booster so that booster and deploying the gas inflator. The booster and autoigniter are separate auxiliary components to the main gas generant, and as such, prior compositions have not significantly contributed to the overall gas generated.
The use of separate booster and autoignition components is problematic for several reasons. Not only does this complicate the manufacturing process by including an additional sub-process and step, but the installation of a separate autoignition cup sub-assembly is also required. Furthermore, using separate autoignition and booster chemical compositions inhibits design flexibility and increases the gas inflator volume.
The hazardous and complicated nature of processing the generant compositions due to their inherent sensitivity to impact and shock presents yet another concern. Often, a component of the mixture is highly explosive leading to further processing precautions. If an autoignition composition were provided without these disadvantages, simplified processing methods such as dry grinding and pelletizing could be used.